1. Field of the Invention
The present invention relates generally to the fabrication of semiconductor devices and, more particularly, to an apparatus and method for the rapid thermal processing of semiconductor wafers.
2. Description of the Related Art
The deposition (or growth) of a film such as, for example, a dielectric material, on the surface of a semiconductor wafer is a common step in semiconductor processing. Such depositions (or growths) are usually performed in an apparatus called a deposition reactor, which generally includes a reaction chamber, a wafer holder and handling system, a heat source and temperature control, and a gas delivery system (e.g. inlet, exhaust and flow control). For example, with reference to FIG. 1 there is shown a simplified cross-sectional view of one type of deposition reactor 100, known as a horizontal furnace, in which a susceptor 101 (wafer holder) is positioned in a horizontal tube 102 (usually of circular cross-section), the interior of which is the reaction chamber. The term reaction chamber as used in this disclosure refers to the area within a reactor where the deposition (or growth) of a film on the surface of a semiconductor wafer occurs. Semiconductor wafers 103 are mounted on surface 101a of susceptor 101. Heat source 104 heats the semiconductor wafers 103 and susceptor 101, and reactant gases 105 are introduced into, and flow through horizontal tube 102, past the wafers 103. Susceptor 101 is often tilted so that susceptor surface 101a faces into the flow of reactant gases 105, reducing the depletion of reactant gases in the vicinity of each semiconductor wafer, especially for those semiconductor wafers located the farthest distance away from the input flow of reactant gases 105.
A typical deposition step for a reactor such as, for example, a horizontal furnace, includes the mixing of selected chemical gases in the gas delivery system, which are subsequently introduced into the reaction chamber for deposition as a layer or film of material, onto the surface of a semiconductor wafer. The heat source heats the reaction chamber and accelerates the chemical reaction of the gases in the chamber and also raises the temperature of the semiconductor wafer, to the particular temperature, necessary for film deposition.
Deposition reactors are also classified according to the characteristics of their operation such as, for example, a rapid thermal process (RTP) reactor which is characterized by the amount of time required for heating the semiconductor wafer during a deposition step, as well as the time necessary for cooling the same semiconductor wafer after a film of material has been deposited on it. The term rapid thermal process as used in this disclosure refers to a process having a heat-up rate of at least 20 degrees Centigrade per second and a corresponding cool-down rate of at least 10 degrees Centigrade per second. Conventional furnace reactors generally require on the order of several hours for both the heating up and cooling down steps necessary for depositing a layer of material on a batch of wafers. In contrast, rapid thermal process (RTP) reactors require only between about 5 seconds to 15 minutes for both the heating up and cooling down steps necessary for depositing a layer of material on a wafer, since the heat source used for heating the semiconductor wafers and accelerating the chemical reaction of the gases in such reactors, are high powered lamps. Thus, rapid thermal process (RTP) reactors are characterized in that the process cycle time for depositing a film of material on a semiconductor wafer is considerably shorter than the process cycle time for the same film of material deposited in a conventional furnace reactor.
For most deposition processes, it is desirable to maximize semiconductor wafer throughput (e.g., the number of wafers processed per unit time), while depositing material layers that have uniform thicknesses. To obtain material layers having uniform thicknesses, it is necessary to maintain the semiconductor wafer at a constant temperature, during the film deposition.
A particular problem with rapid thermal process (RTP) reactors is that it is difficult to maintain the semiconductor wafer at a constant temperature while heating with a high powered light source, due to the support mechanism used for holding the semiconductor wafer during the deposition of the material layer. For example, with reference to the furnace reactor of FIG. 1, the semiconductor wafers are typically mounted on the top surface of the susceptor so that the bottom surface of the semiconductor wafer is in full contact with the top surface of the susceptor (side 103b of semiconductor wafer 103 is in full contact with side 101a of susceptor 101). However, in rapid thermal process (RTP) reactors, the process cycle time is only on the order of a few seconds for some heating and cooling cycles, so that if one side of the susceptor is in full contact with one side of the semiconductor wafer, the deposition temperature and cycle time are difficult to achieve. This is because the mass of the susceptor functions as a heatsink and absorbs heat away from the semiconductor wafer, so that the wafer heats to the deposition temperature at a slower rate, increasing the time required to deposit the layer of material on the surface of the wafer. Conversely, the mass of the susceptor also does not dissipate heat quickly, so that the semiconductor wafer cools at a slower rate, increasing the time needed to cool the wafer after deposition. As a result, the susceptor used for holding the semiconductor wafer in most rapid thermal process reactors supports just the outer periphery of the wafer, instead of the entire surface, in order to reduce the time required to deposit a material layer on the surface of the wafer.
FIG. 2A shows a side view of a typical susceptor used in a rapid thermal process reactor, where just the outer periphery of semiconductor wafer 201 is supported with a non-reactive material 202. The term non-reactive material refers to a material which is chemically inactive with all other materials, including reactant gases used in conjunction with the rapid thermal process reactor. FIG. 2B illustrates a top view of the susceptor shown in FIG. 2A. Area 205, whose outline is shown in phantom (with dashed lines), represents that area on the outer periphery of the semiconductor wafer 201 which is supported by the non-reactive material 202. Area 206 represents that area of the semiconductor wafer 201 which is not supported by the non-reactive material 202. The width of the semiconductor wafer 201, supported by the non-reactive material and represented by area 205, is typically on the order of 1 mm (millimeter).
While the use of the susceptor depicted in FIGS. 2A and 2B, reduces the cycle time required for heating and cooling the semiconductor wafer, area 205, supported by the non-reactive material 202, again acts as a heat sink, causing the outer periphery of the semiconductor wafer to have a different surface temperature during the deposition of the film, than area 206 which is not supported. This forms a temperature gradient in the semiconductor wafer and causes the semiconductor wafer to have different deposition rates for the material layer in the center of the wafer than near the perimeter. As a result, films of material deposited on the surface of the semiconductor wafer have areas with different thicknesses. In addition, if the semiconductor wafers have diameters that are larger than about three inches, warping occurs during the heating and cooling cycles, due to the uneven wafer support provided by the non-reactive material. Thus, films deposited on semiconductor wafers using rapid thermal process techniques have proven to be unsatisfactory, since there is a lack of uniformity in the thicknesses of the films deposited on the surfaces of individual wafers, as well as warping of the wafers. Accordingly, rapid thermal process techniques that maintain the semiconductor wafer at a constant temperature during film deposition and which do not warp the wafers, continue to be sought.